Cobalt-Promoted Tandem C(sp²)–H Activation and C–C Coupling of Vinyldiphenylphosphine: σ- vs π-Coordination Modes within Cyclometalation

Abstract

We have studied the reactivity of a series of terminal alkenyldiphenylphosphines {Ph2P­(CH2) n -CHCH2), n = 0, 1, 2, 3} under mild conditions (−70 °C) in reaction with univalent Co­(I). Methane and ethane elimination occurs when different olefinic P,C-coordination modes are established. Subsequent C–H activation and C–C coupling take place in the reaction of Co­(CH3)­(PMe3)4 with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic P,C,P-type ligand with asymmetric ring sizes attached to the cobalt center of (PMe3)2Co­{κ3-(P,C,P)-(Ph2)­CH2CHCH-CH­(PPh2)} (1). X-ray study of 1 revealed that the complex has a unique cyclometalated structure of a three-membered ring within the newly assembled bis-phosphine ligand. Cyclometalation via selective vinyl-group C­(sp2)–H activation of stoichiometric amounts allyl-diphenylphosphine with Co­(CH3)­(PMe3)4 afforded for the first time σ-allyl coordination incorporated in a five-membered cobaltacycle of (PMe3)2Co­{κ2(P,C)-HCCHCH2PPh2} (2); subsequent C–C coupling is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine and Co­(CH3)­(PMe3)4 afforded a η3-coordinated π-allyl complex of composition (PMe3)2Co­{η3-κ4-(P,C,C,C)-H2CCH-CH­(CH2)2PPh2} (3). All unprecedented complexes were characterized by multinuclear NMR spectroscopy and X-ray diffraction, and their mechanisms of formation are discussed.

Cobalt-Catalyzed Allylic C(sp3)–H Carboxylation with CO2

“…However, these methods do not meet the requirements of our planned carboxyl­ation, since nucleo­philic allyl­metal species are necessary for the reaction with CO 2 . We thus envisaged the generation of low-valent allyl­metal species through the cleavage of allylic C­(sp 3 )–H bonds by low-valent alkyl­metal complexes (e.g., alkyl-Co­(I) and Rh­(I)) with the release of an alkane. , Although the reactivities of these allyl­metal complexes have not been investigated, we hypothesized that a low-valent allyl­metal complex shows high nucleo­philicity toward CO 2 based on the precedented transition-metal-catalyzed carboxyl­ation, furnishing β,γ-unsaturated carboxylic acids (Scheme , eq 2).…”

“…Subsequently, two different ways are possible. First, reductive elimination of methane from IV would lead to a low-valent allyl-Co species V , which is stabilized by an aryl or alkenyl ligand . C–C bond formation with CO 2 would proceed at the γ-position to produce Co carboxylate VI .…”

Co-Catalyzed Direct Addition of Allylic C(sp3)–H Bonds to Ketones

“…To develop a new electrophilic transformation of the allylic C­(sp 3 )–H bonds of alkenes, we considered generating a low-valent η 3 -allylmetal complex. Alkylated transition-metal species such as alkylcobalt­(I) and alkylrhodium­(I) were reported to cleave the allylic C–H bond of alkenes to generate an η 3 -allylmetal­(I) complex with the release of an alkane . However, the reactivities of these complexes were not fully studied, which attracted our initial interest.…”

3d Transition Metals for C–H Activation

“…The size of the formed ring could even be reduced to strained 4-membered rings, as was shown for phosphines, , thioles, and thioethers 409 . Further, Beck and Klein demonstrated a shift in the binding mode from σ- to π-C–Co bond when performing cyclometalation with vinyl-, allyl-, and homoallylphosphanes …”